Most living organisms on earth are adapted to a 24 hr cycling environment due to the rotation of the earth. An organism knows the time of the day and prepares the incoming abiotic and biotic stresses, and also utilizes the expected changes of ambient environment. This time-measuring mechanism in an organism is an important biological function for its survival in nature. The observable biological rhythm with about a 24 hr period is collectively called circadian rhythm, and the underlying molecular mechanism of circadian rhythm is called the circadian clock. It has been suggested in different model organisms that circadian rhythm is a fitness trait. However, we still lack knowledge in regards to how circadian clock achieves the fitness advantage. Researchers in the proposal have identified Continent-Specific-Clock-Variation in Neurospora discreta species complex Phylogenetic Species 4b (PS4b) in strains collected in North American strains in comparison to strains collected from other continents, and proposed to characterize how allelic variations of the adaptive gene causes variation of the clock phenotype to adapt to their unique habitat using genetics, genomics, proteomics, and computational biology tools. They will generate allele-swap strains where their genotype is identical, except the continent-specific allele of the candidate gene. The fitness value of the candidate gene will be tested by measuring the number of progeny under different habitat conditions. The successful outcome of the proposed study will allow us an insight on how a local adaption of circadian rhythm of an organism influences the organism's fitness.